The RS-68A is the world’s most powerful liquid-hydrogen/liquid-oxygen booster engine designed to provide increased thrust and improved fuel efficiency for the Delta IV family of launch vehicles.
It is evolved from the RS-68 engine, which was developed and certified for commercial use with private company funds. Each RS-68A provides 702,000 pounds of lift-off thrust, or 39,000 more pounds of thrust than the RS-68 engine, with increased combustion efficiency as well.
During hot-fire tests, the RS-68A engine demonstrated the ability to operate for 4,800 seconds of cumulative run time – more than 10 times what's needed to boost the Delta IV Heavy rocket into space.
In June 2012, three RS-68A engines thundered into the sky on their inaugural launch, boosting a U.S. government satellite onboard a United Launch Alliance Delta IV Heavy rocket from Cape Canaveral Air Force Station in Florida.